The penetrant method of non-destructive testing is based on capillary penetration of indicator liquid (penetrant) into the surface defects (cracks, pores, etc.). After intermediate cleaning and subsequent drying, a developer is applied as a thin, white layer onto the surface to be inspected. The developer “pulls” the penetrant out of the surface defects and makes them visible as coloured, linear or rounded indications. Documentation is possible by means of adhesive films, photographs or video techniques. Penetrant testing is reliable in detecting the defects with a width down to the range of μm.

Penetrant inspection can detect surface defects accurately and easily independent of types, materials and surface configuration of the objects to be tested. Other methods of non destructive testing can be applied only with restrictions. Necessary condition: defects must be open to surface. Capillary systems have been also used for the leak testing.